The present invention relates to a method of interference suppression in a radar system. The invention also relates to a radar system operating according to the method.
In a convenonl radar system, an antenna assembly is employed to emit and receive radar radiation, such radiation being defined as electromagnetic radiation typically in a frequency range of 3 MHz to 100 GHz. The antenna assembly exhibits a non-ideal polar response comprising a direction of relatively higher gain and number of sidelobes which lie in directions noncoincident with the direction of higher gain. Radiation emitted from the assembly is principally concentrated in the direction of higher gain, hereinafter referred to as a main beam direction of the system, with residual radiation distributed in sidelobe directions. Likewise, radiation received at the assembly is most sensitively received in the main beam direction and relatively less sensitively received in the sidelobe directions. In the polar response, a number of nulls separate the main beam from the sidelobes and also mutually separate the sidelobes.
In the conventional system, the assembly can comprise a plurality of antenna elements whose input and output signals corresponding to radar radiation emitted and received thereat respectively. The assembly is electronically steerable by individually scaling and phase shifting the signals to and from the elements. Alternatively, the assembly can comprise a single relatively larger antenna element which is mechanically scanned in a field of view of the system. In some conventional systems, a combination of mechanical and electronic steering is employed.
Although an ideal radar system includes only a main beam in its antenna assembly polar response, technical design limitations, for example practical radar antenna aperture size limitations, mean that conventional radar systems exhibit sidelobes in their polar responses. Such sidelobes make the conventional radar systems susceptible to sources of interference lying in directions corresponding to the sidelobes. The sources of interference can comprise other radar systems for example.
There are several methods presently employed to counteract effects of interfering sources in conventional radar systems. These methods are described in a widely available published book xe2x80x9cAntenna-Based Signal Processing Techniques for Radar Systemsxe2x80x9d by Alfonso Farina, published by Artech House ISBN 0-89006-396-6, 1992.
A first well known conventional method to counteract interference is known as sidelobe blanking (SLB). In this first method, a conventional radar system employs a first antenna assembly exhibiting a relatively angularly narrow main beam to interrogate its field of view and receive corresponding echo radiation therefrom; angularly narrow in this context means typically in the order of 1xc2x0 between response xe2x88x923dB points. The system additionally employs a second antenna assembly exhibiting a relatively much broader main beam having less gain compared to the narrow main beam to receive the echo radiation; much broader is this context means typically in the order of 20xc2x0 between response xe2x88x923dB points. For each direction in the field of view in which the antennae are scanned, a first signal corresponding to the radiation received at the first antenna is compared with a second signal corresponding to the radiation received at the second antenna For a given direction in the field of view, the first signal is considered to come from a sidelobe direction if the second signal is greater in magnitude than the magnitude of the first signal multiplied by a predetermined factor.
In the first method, the system can incorporate two separate antenna assemblies for generating the narrow main beam and the much broader main beam. Alternatively, the system can derive the narrow and broader beams from a single multielement antenna assembly by suitably combining in phase and amplitude signals generated by the elements.
SLB is effective at identifyg interference from pulsed interference souses in the sidelobe directions. However, SLB suffers a problem that a radar system employing it is effectively blind to real targets whose radar echoes arrive at an identical time to that of an interfering source. Thus, an interfering source which is able to generate a number of false targets at different times of arrival at a radar system employing SLB is capable of masking real targets over a substantial part of the field of view.
A second well known conventional method of counteracting interference is known as adaptive beam forming and has been widely reported in scientific literature and also in the aforementioned book. In the second method, a conventional radar system comprises a multielement antenna assembly. Radar radiation received and emitted at each element gives rise to corresponding output and input signals respectively. The output signals are coherently combined to create a composite received signal corresponding to a composite radar receive main beam which has nulls steered in directions of sources of interference within the field of view of the system.
When coherently combining the output signals to generate the composite signal in the second method, the signals are manipulated in relative phase and amplitude by weighting coefficients which are calculated within the system. Calculation of the coefficients is mathematically non-trivial and is described in Chapter 4 of the aforementioned book. It involves correlation of sample signals including signal components corresponding to all sources of interference to be eliminated. Moreover, the sample signals must also be substantially free of target and clutter signals which can adversely affect the accuracy to which the coefficients are calculated If target originating signals are present, nulls will be steered in directions of the targets.
The second method suffers a problem that nulls associated with the composite signal are only steered in directions of interfering sources which generate corresponding signal components in the sample signals. Thus, if the interfering sources radiate radar radiation noise continuously, it is likely that components of their radar noise will be present in the sample signals and therefore effectively nulled. However, if the interfering sources radiate radar radiation in the form of periodic pulses, then there is a possibility that components corresponding to the pulses are not included in the sample signals and are thereby not effectively nulled.
Thus, there are situations where the first and second methods are unable to effectively counteract the effects of interfering sources within the field of view of the conventional system.
The inventor has appreciated that the problems described above can be addressed by an alternative method of operating a radar system which synergistically combines aspects of the first and second methods.
According to a first aspect of the present invention, there is provided a method of noncontinuous jamming interference suppression in a radar system incorporating emitting means for emitting interrogating radiation towards a remote scene and receiving means for receiving corresponding echo radiation returned from the scene in response to the interrogating radiation, the method including the steps of:
(a) emitting the interrogating radiation from the emitting means towards a selected region of the scene;
(b) receiving first echo radiation substantially from the selected region of the scene at the receiving means and generating a corresponding first received signal;
(c) receiving second echo radiation substantially from the selected region and other regions surrounding the selected region and generating a corresponding second received signal;
(d) mutually comparing the first and second signals and determining therefrom portions of the first signal subject to noncontinuous jamming interference;
(e) repeating steps (a) to (d) where necessary for one or more other selected regions until sufficient samples of the signals are available for performing an adaptive weight calculation;
(f) selectively using the portions of the first signal where non-continuous jamming interference is detected in the calculation for calculating adaptive weight coefficients;
(g) processing the signals using the adaptive weight coefficients to generate an overall output from the system corresponding to radiation reflected from one or more selected regions, the overall output being at least partially corrected for noncontinuous jamming interference enabling target detection during periods of such interference; and
(h) repeating steps (a) to (g) until all the selected regions of the scene to be interrogated have been interrogated.
The invention provides the advantage that the method makes the system capable of employing a relatively narrower receiving means polar response in step (b) and a relatively broader receiving means polar response in step (c) to identify in step (d) those signals subject to interference and thereby use portions of the signals subject to interference selectively when calculating the adaptive weight coefficients in step (I) to enhance interference suppression provided by the system.
Advantageously, to improve interference suppression provided by the system operating according to the method, signals are stored in storing means of the system for a period not less than that required for updating the adaptive weight coefficients. Storing the signals enables more representative samples to be used when calculating the adaptive weight coefficients.
Conveniently, for determining whether or not interference is present, the method can exploit a condition where the second signal is greater in magnitude than the magnitude of the first signal subject to a threshold scaling constant for a given selected region as being indicative of the first signal being affected by interference. Advantageously, the scaling constant can be made variable for coping with different types of interference, for example pulsed interference as compared to continuous interference.
Beneficially, in order to provide a more agile radar system, the method can employ electronic steering in steps (b) and (c) for steering the receiving means to different selected regions of the scene. Additionally, in order to obtain a wide coverage of the scene, the method can employ mechanical steering to steer the receiving means to different selected regions.
Conveniently, the receiving means can comprises a multielement antenna and the adaptive weight coefficients can be used to vectorially multiply signals generated by the elements in response to radiation received from the scene thereat for generating the overall output from the system. Multielement antennae are susceptible to electronic steering and hence are capable of imparting greater agility to the radar system when incorporated therein compared to purely mechanically steered single element antennae incorporated therein.
In a second aspect, the invention provides a radar system operating according to the method of the first aspect of the invention.
An embodiment of the invention will now be described, by way of example only, with reference to the following diagrams in which: